In the case of the ink-jet printing process, tiny ink droplets are applied by means of different techniques, which have already been repeatedly described, onto a recording material and absorbed by the latter. The recording material is subject to different requirements such as a high colour density of the printed dots, a high ink absorption capacity, a short drying time and a sufficient smudge resistance associated therewith, a dye diffusion in the transverse direction of the printed dots not exceeding the required extend (bleed) as well as a low color coalescence and a high resistance to water. Other requirements, in particular for photograph-type prints, are an even print gloss and surface gloss of the recording material.
As a result of the great progress achieved in the ink-jet technology field regarding the quality of the image and the speed of printing of the printer, the ink-jet color printer market has grown enormously. It is expected that, in the next few years, the area of digital photography, in particular of ink-jet materials of photographic quality, will continue to develop strongly. Due to the need to achieve a photographic image quality in the case of ink-jet printed products, achieving a high image durability is one of the most important development tasks. This leads to new developments both regarding the image-receiving materials and the dye-based inks.
Ink-jet recording materials can be divided into two classes: materials with recording layers swelling in water which contain binders such as gelatine or polyvinyl alcohol, and materials with microporous layers.
Materials with layers swelling in water have the advantage that they are glossy and exhibit high color densities after printing. They exhibit a satisfactory stability vis-à-vis ozone since the swellable layers—like a barrier—prevent ozone from penetrating into the material. However, the material has disadvantages regarding the image quality (bleed, color coalescence) and the drying time.
The microporous systems, which are capable of rapidly absorbing the ink during printing as a result of cavities in the applied layer, exhibit an excellent image quality as a result of the good color fixing effect. They have a short drying time and problems regarding coalescence and bleed do not occur. The images produced by means of such microporous recording materials are not resistant to light and react in a sensitive manner to the gases contained in the air, particularly ozone. The gas can easily penetrate into the micropores of the recording layers and—encouraged by the catalytic effect of the pigments contained in the layers—possibly attack the double bonds of the dyes. The reactivity vis-à-vis ozone can be further increased by moisture at elevated temperatures. Cyan blue dye has a particularly sensitive reaction to ozone.
The solutions to the problem suggested at present include laminating of the printed image with a polyester film or the use of inks containing alkali metal additives and ammonium or amine salt groups. Although these solutions have positive aspects, they are not free from disadvantages. The disadvantages include, on the one hand, an increase in the production costs caused by the laminating step; on the other hand, the metal-containing compounds tend to cause significant tint shifts which become noticeable as color turbidity.
According to JP 10-264501, the stability vis-à-vis ozone can be improved by using a plasticiser-containing thermoplastic resin as binder in the ink absorption layer. The anti-ozone-protecting effect is presumably achieved by the pigment and the absorbed ink dyes being enveloped by the resin.
A further possibility of improving the stability vis-à-vis ozone is described in JP 08-164664 where an inorganic pigment, the surface of which is modified with cycloamylose, is used in the ink absorption layer.
In EP 0 524 635 A1, a recording material is suggested which contains a combination of starch particles, an ethylene-vinyl acetate copolymer and a cationic dye-fixing agent in the ink absorption layer.
According to U.S. Pat. No. 6,344,262 B1 an Mg thiocyanate is used in a porous layer containing aluminium oxide in order to improve the stability vis-à-vis light and ozone. The porous layer is the ink-recepting layer.
In EP 1 157 847 A1, the use of benzotriazol derivatives in the ink absorption layer for improving the resistance of the recording material to gas is described.